Stable carbon isotope ratios of ethane over the North Pacific ...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D02308, doi:10.1029/2010JD014602, 2011

Stable carbon isotope ratios of ethane over the North Pacific: Atmospheric measurements and global chemical transport modeling Takuya Saito,1,2 Olaf Stein,3 Urumu Tsunogai,4 Kimitaka Kawamura,1 Takeshi Nakatsuka,1,5 Toshitaka Gamo,4,6 and Naohiro Yoshida7 Received 9 June 2010; revised 8 November 2010; accepted 22 November 2010; published 27 January 2011.

[1] The atmospheric mixing ratios of ethane and its stable carbon isotope ratios (d 13C) were

measured over the North Pacific (2°N to 38°N, 140°E to 90°W) during oceanographic cruises in summer and autumn. The measured mixing ratios were relatively low (mostly 1 ppbv) were observed over the western North Pacific near Japan, with lower d 13C values (approximately −25‰), suggesting recent emissions from neighboring source regions. The most 13C‐enriched values of ethane (approximately −16‰) were observed over the western equatorial Pacific rather than the central and eastern equatorial Pacific. This is likely caused by the kinetic isotope effect (KIE) for the removal of ethane during the atmospheric transport from potential upwind source regions to the most remote region under the prevailing trade easterly winds. The measurements were compared with the results of a global chemical transport model including two ethane isotopologues (12C2H6 and 13C2H6). The model‐estimated d13C values were too high compared with the observations. It is likely that this discrepancy is partly due to an approximately 40% overestimation of the reported KIE for the reaction between ethane and OH radicals. Citation: Saito, T., O. Stein, U. Tsunogai, K. Kawamura, T. Nakatsuka, T. Gamo, and N. Yoshida (2011), Stable carbon isotope ratios of ethane over the North Pacific: Atmospheric measurements and global chemical transport modeling, J. Geophys. Res., 116, D02308, doi:10.1029/2010JD014602.

1. Introduction [2] Ethane, the second most abundant hydrocarbon in the remote atmosphere after methane, acts as a precursor of ozone and other photochemical oxidants (e.g., acetaldehyde and peroxyacetyl nitrate) through photochemical degradation, mainly by OH radicals. The atmospheric lifetime of ethane (∼2 months [Rudolph, 1995]) is long enough for it to undergo long‐range transport from source regions to remote areas but short enough for ethane mixing ratios to vary substantially both spatially and temporally. The mixing ratios of ethane, in combination with those of other nonmethane 1 Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan. 2 Now at Environmental Chemistry Division, National Institute for Environmental Studies, Ibaraki, Japan. 3 Institut für Energie‐ und Klimaforschung – Troposphäre (IEK‐8), Forschungszentrum Jülich, Jülich, Germany. 4 Graduate School of Science, Hokkaido University, Sapporo, Japan. 5 Now at Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan. 6 Now at Ocean Research Institute, University of Tokyo, Tokyo, Japan. 7 Department of Environmental Science and Technology, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama, Japan.

Copyright 2011 by the American Geophysical Union. 0148‐0227/11/2010JD014602

hydrocarbons, are therefore useful indicators of atmospheric oxidation and transport processes on regional, hemispheric, and global scales [Blake et al., 1996a; Goldstein et al., 1995; Helmig et al., 2008; Parrish et al., 1992]. Ethane can also be used as a unique tracer that provides constraints on the fossil fuel sources of methane [Xiao et al., 2008]. [3] Stable carbon isotope ratio (d13C) measurements of ethane can provide additional insight into its sources, sinks, and distributions in the atmosphere. Although the number of such studies is still quite limited [e.g., Nara et al., 2007; Rudolph et al., 1997; Tsunogai et al., 1999], a few studies [Saito et al., 2002, 2009] have shown that atmospheric d 13C measurements of ethane can be used to estimate the extent of photochemical aging within the theoretical framework of the “isotopic hydrocarbon clock” [Rudolph and Czuba, 2000]. Another approach to the interpretation of d13C observations is to use a numerical model. Volatile organic compounds (VOCs), which have relatively long lifetimes (approximately >1 year) and are well mixed in the atmosphere, allow the use of a simple isotope mass balance model to constrain the atmospheric budget [Bill et al., 2004; Keppler et al., 2005; Saito and Yokouchi, 2008; Thompson et al., 2002]. In contrast, more reactive VOCs, such as ethane, require an isotope‐ inclusive global chemical transport model. [4] Thompson et al. [2003] have developed such a global chemical transport model for ethane and benzene; their model

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treats the emissions and chemistry simply by using a uniform stable carbon isotope ratio for all sources and a prescribed OH radical concentration. A more comprehensive model with source‐specific isotope ratios of ethane and detailed model chemistry has been presented by Stein and Rudolph [2007]. These studies have demonstrated that comparison of atmospheric d13C measurements with estimates from a chemical transport model can provide a useful test of our understanding of the emissions, atmospheric transport processes, and chemical reactions of VOCs. However, to our knowledge, no study has directly compared the ambient d 13C measurements and model results corresponding to the temporal and spatial observations for specific years. [5] Here, we present stable carbon isotope measurements of ethane in the marine atmosphere during two oceanographic cruises over the North Pacific. We compare these with the results from a three‐dimensional chemical transport model, and we discuss the factors controlling the d13C of ethane in the background marine atmosphere.

2. Experimental Procedure [6] Eighty‐eight air samples were collected over the North Pacific during two cruises: (1) the trans‐Pacific cruise of R/V Shoyo‐maru (Japan Fisheries Agency) between Japan and Mexico in September–October 1999 (40 samples, 2°N to 35°N, 145°E to 90°W) and (2) the western North Pacific cruise of R/V Mirai (Japan Marine Science and Technology) in June–July 2000 (48 samples, 7°N to 38°N, 140°E to 142°E). Air samples were taken on the front of the upper deck using pre‐evacuated 6 L fused‐silica‐lined stainless steel canisters (Silico‐can, Restek Co., Ltd.), which were kept until the end of the cruises before being transported to the laboratory for the analysis. [7] Stable carbon isotope ratios of ethane were measured with a cryogenic vacuum extraction line and by gas chromatography – combustion – isotope ratio mass spectrometry (GC‐C‐IRMS) in accordance with the method of Rudolph et al. [1997]. Details of the experimental setup have been documented by Tsunogai et al. [1999]. Briefly, each air sample (approximately 6 L) was condensed in a three‐stage preconcentration process in a bath of liquid N2. During the procedure, bulk air gases (mostly N2 and O2), water vapor, and CO2 were removed. The purified air sample including ethane was then introduced to the gas chromatograph equipped with a PoraPLOT‐Q capillary column (25 m long × 0.32 mm internal diameter) after a cryofocusing of ethane and other hydrocarbons at liquid N2 temperature at the head of the capillary column. The stable carbon isotope ratios of the ethane were measured by IRMS (Finnigan MAT 252) in continuous flow mode, following combustion of the ethane to CO2. [8] The accuracy of the isotopic measurements was estimated to be better than 0.3‰ versus Peedee belemnite (PDB) by the measurement of a U.S. National Institute of Standards and Technology RM 8560 (International Atomic Energy Agency (IAEA) NGS2) isotopic standard. The reproducibilities derived from repeated analyses of a working standard were less than 0.5‰ for GC injection of >200 pmol C,